The present embodiments relate to a magnetic resonance tomography system.
An MRT system and a corresponding receive apparatus are known from DE 199 11 988 A1, for example.
A system for creating magnetic resonance tomograms may be formed from at least two parts separated spatially from one another. The first part includes a device for generating a magnetic field that may create a magnetic resonance in a body to be examined (e.g., a body of a human or animal patient) or in a material sample. It is difficult to operate complex electronic circuits or even computer systems for the computation of tomograms close to the body. These systems interfere with the receive apparatus, with which the magnetic resonance signal (e.g., MR signal) emitted as a result of the magnetic resonance is received. This area, in which the use of potential interfering circuits is to be avoided, is referred to as the measurement field. To process the MR signal (e.g., for editing the image information contained therein), the signal is transmitted out of the measurement field to an evaluation device (e.g., a computer system) that is located in an area shielded as regards RF (e.g., from which no interfering high-frequency radiation (RF radiation) may get into the measurement area). The spatial distances to be spanned in such cases may amount to 5 m and more.
The MR signal contains the desired image information in a specific frequency band, the mid frequency of which depends on a field strength of the basic magnetic field or B0 magnetic field. A bandwidth of the frequency band is produced by the gradient fields that are generated by gradient coils of the MRT system. For digital editing of the MR image information by the evaluation unit, the MR signal or a signal derived therefrom are to be converted into a digital signal using an analog/digital converter.
It is known from the above publication that an analog/digital converter may be placed close to the receive coil in such cases, and the high-frequency MR signal may be sampled directly in this way in order to reduce the analog circuit outlay. The digital sampling values obtained by the A/D converter may then be exported from the magnetic field, for example, via a serial data link to the evaluation device. However, high A/D converter sampling rates are required for A/D conversion to obtain a high decimation gain (e.g., a marked oversampling of the analog signal by a factor of 2 and greater). These high A/D converter sampling rates require a correspondingly high data transmission rate, which is used for transmission out of the magnetic field. With the ever-increasing field strengths of the basic magnetic field of current MRT systems, however, a mid frequency of the MR signal that may lie in the range of 40 MHz to 500 MHz is produced. Correspondingly fast A/D converters make the manufacturing of the MRT system very expensive. In addition, the long communication links between the A/D converter and the RF-shielded evaluation unit provide that corresponding high-frequency interference signals, which may only be suppressed at great expense, are emitted by the connecting lines.
To obtain the MR image information from the MR signal, the frequency band containing the image information is shifted into a baseband, where the image information may be further processed as a low-frequency signal. A digital mixing down into the baseband close to the examination magnetic field (e.g., within the receive apparatus) is not sensible, since a control connection from the evaluation unit is needed in order to be able to adapt the sampling rate of the A/D converter to the measurement sequence currently being used. The control signal may be emitted from the control connection, may couple into the receive coil element of the receive apparatus via an electromagnetic coupling-in, and may disrupt the MR signal there.